Fix for `isRequiredFor` is not a function

[bkeepers]

This adds an integration test and includes a fix for isRequiredFor is not a function which was one of the causes for #69

[ci-reporter[bot]]

The build is failing

✨ Good work on this PR so far! ✨ Unfortunately, the Travis CI build is failing as of 0e5ae9ae2b1038429d33ef919f626e5af4dc72c1. Here's the output:

npm test

> probot-dco@1.0.0 test /home/travis/build/probot/dco
> jest && standard

 PASS  test/dco.test.js
 FAIL  test/index.test.js
  ● Test suite failed to run

    /home/travis/build/probot/dco/node_modules/probot/lib/index.js:51
          ...event,
          ^^^
    SyntaxError: Unexpected token ...

      1 | const nock = require('nock')
      2 | const app = require('..')
    > 3 | const createProbot = require('probot')
      4 | const payload = require('./fixtures/pull_request.opened')
      5 | const compare = require('./fixtures/compare')
      6 | 

      at ScriptTransformer._transformAndBuildScript (node_modules/jest-runtime/build/script_transformer.js:316:17)
      at Object.<anonymous> (test/index.test.js:3:22)

Test Suites: 1 failed, 1 passed, 2 total
Tests:       17 passed, 17 total
Snapshots:   0 total
Time:        3.649s
Ran all test suites.

I'm sure you can fix it! If you need help, don't hesitate to ask a maintainer of the project!

---

Failed build for 81d4e9c

##### `npm test` ``` > probot-dco@1.0.0 test /home/travis/build/probot/dco > jest && standard PASS test/dco.test.js FAIL test/index.test.js ● Test suite failed to run /home/travis/build/probot/dco/node_modules/probot/lib/index.js:51 ...event, ^^^ SyntaxError: Unexpected token ... 1 | const nock = require('nock') 2 | const app = require('..') > 3 | const createProbot = require('probot') 4 | const payload = require('./fixtures/pull_request.opened') 5 | const compare = require('./fixtures/compare') 6 | at ScriptTransformer._transformAndBuildScript (node_modules/jest-runtime/build/script_transformer.js:316:17) at Object. (test/index.test.js:3:22) Test Suites: 1 failed, 1 passed, 2 total Tests: 17 passed, 17 total Snapshots: 0 total Time: 4.553s Ran all test suites. ```

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[github-actions[bot]]

:tada: This PR is included in version 1.0.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: